The receptive relaxation reflex [RRR] is acknowledged as one of the most important vagal reflex mechanisms coordinating feeding behavior with gastric function. In the classic description of the reflex, distension of the esophagus, as occurs while feeding, causes a vagally-mediated relaxation and suppression of motility. This reflex can be elicited experimentally and is assumed to allow the stomach to accept ingesta isobarically. More recent reports show, however, that the RRR must be some how suspended, or even "inverted" during and after feeding since gastric transit is actually more rapid at these times than during fasting. Our own studies show that in rats fasted overnight, esophageal distention causes (as expected) a powerful gastric relaxation. Conversely, our observations in fed rats show that esophageal distention induces a significant increase in gastric tone. This feeding-related increase in gastric tone induced by esophageal distention gradually reverts back to a "pure" gastric relaxation after few hours. This dramatic change in the function of such an essential vagal control reflex provides the basis for our overarching hypothesis: Neural circuits underlying vago-vagal gastrointestinal control reflexes are highly dynamic and regulated by factors associated with feeding status. We have previously shown that extrinsic factors, especially gastrointestinal hormones, directly and dramatically modulate vago-vagal reflex control of the gut. However, the specific mechanisms by which Gl hormones can reorganize a reflex response to a visceral afferent input are not well understood. We propose that a subset of gut hormones, in particular, CCK and GLP-1, act on neurons in the dorsal vagal complex to alter the availability of presynaptic catcholamine receptors in the solitary nucleus [NTS]. We propose further that this shift in receptor population has a dramatic effect on reflex neurotransmission patterns in the dorsal vagal complex. We will apply a combination of neurogastroenterological, neurophysiological and immunohistochemical methods to test these proposals. Results from these studies on mechanisms regulating the feeding-state-related switching of vago-vagal reflexes could also help explain how changes in metabolic and hormonal parameters can act to switch ingestion program generation circuits in the brainstem from a "feeding" to "not feeding" state.